Fourier transform infrared spectroscopy, temperature-programmed desorption, and X-ray photoelectron spectroscopy have been used to study the chemisorption and decomposition of trimethylaluminum (TMA) on silica under high vacuum. By annealing a series of silica substrates from 425 to 1573 K prior to TMA exposures at 300 K, we have examined the distributions of chemisorption products as a function of the relative concentrations of isolated hydroxyls (OHi), hydrogen-bonded hydroxyls (OHH), and siloxane bridges. The observed variation in the Si-methyl to AI-methyl population ratios supports a new chemisorption model in which a monomethylaluminum surface complex and methyl groups bonded to silicon are proposed as the majority species on the surface at 300 K. Although the initial reactive sticking probability for TMA on the silica substrates is <0.01 at 300 K, TMA chemisorption affects OHi, OHH, and siloxane bridges on the surface with equivalent probability. The common reaction probability (equivalent rate constants) implies that similar requirements may be involved in the reactive adsorption that consumes each site. Decomposition of the monomethylaluminum adsorbate begins above 373 K and increases the population of methyl groups bonded to silicon on the surface. The methyl groups react to form methane, and ethane and adsorbed hydrocarbon fragments. In addition, the methyl groups also react further with the surface to form tetramethyhane . At coverages of one-third saturation and greater, the monomethylaluminum surface complex can also react with methyf groups to yield TMA, above 500 K.

Co-doped ZnO͑Co x Zn 1−x O͒ is of potential interest for spintronics due to the prediction of room-temperature ferromagnetism. We have grown epitaxial Co x Zn 1−x O films on Al 2 O 3 ͑012͒ substrates by metalorganic chemical vapor deposition using a liquid precursor delivery system. High concentrations of Co͑x ഛ 0.35͒ can be uniformly incorporated into the film without phase segregation. Co is found to be in the +2 oxidation state, independent of x, by both surface-sensitive core-level x-ray photoemission and bulk-sensitive optical absorption spectroscopies. This material can be grown n-type by the deliberate incorporation of oxygen vacancies, but not by inclusion of ϳ1 at. % Al. Semiconducting films remain ferromagnetic up to 350 K. In contrast films without oxygen vacancies are insulating and nonmagnetic, suggesting that exchange interaction is mediated by itinerant carriers. The saturation and remanent magnetization on a per Co basis was very small ͑Ͻ0.1 B /Co͒, even in the best films. The dependence of saturation magnetization, as measured by optical magnetic circular dichroism, on magnetic field and temperature, agrees with the theoretical Brillouin function, demonstrating that the majority of the Co͑II͒ ions behave as magnetically isolated S =3/2 ions.

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The surface coverage of thiolated tetraphenylporphyrin derivatives assembled on gold has been determined using ultraviolet/visible absorption spectroscopy (UV/Vis) and x-ray photoelectron spectroscopy (XPS). Ultraviolet/visible absorption spectroscopy was used to calculate the surface concentration of porphyrin molecules through two independent approaches: the first measured the porphyrin concentration in solution after the displacement of a porphyrin monolayer from the gold surface; and the second directly exploited the absorptivity of the monolayers. Furthermore, we determined experimentally the extinction coefficients for the porphyrin monolayers and verified that the extinction coefficients are in agreement with a simple theoretical model that is frequently applied to porphyrin monolayers. Two separate models, both based on a uniform overlayer, were applied to the XPS data to determine the surface concentration of the porphyrin monolayers. Finally, for comparison of the UV/Vis and XPS results, a model of an ordered tetraphenylporphyrin arrangement on a surface was utilized to calculate coverage. The results of these two spectroscopic techniques differed by as much as a factor of 4, but they were generally comparable to those for related porphyrin systems. Determination of the surface coverage of monolayers is often challenging, thus the comparison of two independent experimental techniques allows for a more accurate estimation of monolayer coverage, and demonstrates the often unrealized potential to calculate the coverage of organic monolayers using XPS.